1. Field of the Invention
The present invention relates to a polishing fluid composition useful for polishing surfaces of semiconductor silicon wafers, or surfaces of films comprising silicon formed on silicon wafers (hereinafter, the term "silicon wafer" also includes such a film). Additionally, the present invention relates to a polishing method using such a polishing fluid composition.
2. Description of the Related Art
Hitherto, alkaline colloidal silica compositions comprising silica abrasives suspended in alkaline solutions have been widely used as polishing fluids for polishing surfaces of semiconductor silicon wafers. Although it is not clear when the use of colloidal silica as abrasives in such polishing fluids started, Japanese Examined Patent Publication No. 49-13665 has already disclosed that silica sol (colloidal silica) can be a suitable abrasive used for obtaining an accurate mirror surface.
Further, "Kikai to Kougu (Tool Engineer)" (August, 1984, pp.38-46, written by Toshiro Karaki) also describes that a mechanochemical polishing method using a soft synthetic leather polisher and an alkaline colloidal silica polishing fluid comprising silica fine particles suspended in an alkaline solution is generally employed for polishing semiconductor silicon wafers, and that such a method includes a mechanical polishing effect by the silica fine particles together with a chemical polishing effect by the alkaline solution, and can achieve a specular surface exhibiting extremely fine smoothness and satisfactory crystallinity.
In general, alkali hydroxides, ammonia, amines, and others are used solely or in combination as an alkaline component used in mechanochemical polishing, and these compounds are added and dissolved in the polishing fluid directly or in the form of salts. Additionally, the pH value of the polishing fluid is ordinarily adjusted to approximately 9 to 12, and solid fine particles such as silica particles or quartz particles having particle diameters of approximately 5 to 300 nm are used as abrasives, and are contained in the polishing fluid at approximately 1 to 50%.
Recently, various techniques in relation to polishing fluids were offered. For example, (1) Japanese Unexamined Patent Publication No. 4-291722 discloses an abrasive containing a nonionic surfactant having a HLB (hydrophilic lipophilic balance) value of 13 to 20, which is directed to obtaining a wafer surface without haze; (2) Japanese Unexamined Patent Publication No. 3-202269 discloses an abrasive comprising colloidal silica with the addition of a disinfectant, which is directed to prevention of bacterial proliferation in the abrasive slurry; and (3) Japanese Unexamined Patent Publication No. 4-63428 discloses an abrasive comprising silica particles and water with the addition of a water-soluble polymer and salts, which is directed to improvement of surface smoothness. Similar to the aforementioned method, these techniques utilize solid fine particles such as silica particles or quartz particles as abrasives.
Although polishing fluids merely utilizing solid fine particles such as zirconia particles or alumina particles as abrasives were also reported previous to the above techniques, almost all recent polishing fluids utilize the principle of mechanochemical polishing in which a mechanical polishing effect by solid fine particles as abrasives and a chemical polishing effect by an alkaline component are combined. Such mechanochemical polishing is employed as a technique capable of satisfying demand for a high removal rate, an excellent surface smoothness and flatness, and negligible processing damage.
Meanwhile, chemical polishing methods using abrasive-particle-free alkaline solutions as polishing fluids are well-known as polishing methods which do not cause processing damage. With only such chemical polishing, however, texture of the polished surface is inferior. In order to compensate for such inferiority, disk-system chemical polishing methods have been proposed (for example, in "Junkatsu (Lubrication)" Vol. 33, No. 4, pp.253-259, written by Toshiro Doi). In such methods, abrasion is carried out only with a chemical solution (polishing fluid) on a soft polisher such as abrasive cloth or a pad having the form of a flat disk. Such methods, however, cannot yet be effectively applied to silicon wafer polishing since polishing rates in these methods are slower than in the aforementioned mechanochemical polishing method using an alkaline colloidal silica composition as a polishing fluid.
As described above, conventional silicon wafer polishing fluids are generally based on the principle of mechanochemical polishing which includes a mechanical polishing effect by abrasive particles and a chemical polishing effect by an alkaline component, and such fluids contain solid fine particles such as silica particles having particle diameters of approximately 5 to 300 nm as abrasives. Accordingly, a large amount of the abrasive particles adhere to and remain on the surface of the silicon wafer polished with such a fluid, and elimination or removal of the residual particles is greatly disadvantageous in view of manufacturing process. In particular, since semiconductor devices have been increasingly densified in recent years, and even a trace amount of residual particles affect performance of produced semiconductor devices, imperfect elimination or removal of residual particles can possibly cause great problems.
Further, when abrasives are carried into a washing facility, other problems can also be caused. For example, the working environment and the apparatuses may be contaminated, the washing solution may become unusable within a short time period, or management of such an environment, apparatuses, and others may become complicated. Besides, the abrasives dispersed in the polishing fluid may separate, precipitate, or secondarily aggregate in the fluid, and therefore, labor is required for storage management of the polishing fluid. Additionally, when the abrasives secondarily aggregate and form larger particle diameters, the surface being polished can be scratched thereby, and in such a case, the polishing fluid cannot serve its original function any longer.
In contrast to such a mechanochemical polishing method, the above-described disk-system chemical method is performed using a polishing fluid substantially free of abrasives, and therefore, can be expected to be a practical method which can be performed without the above-described problems due to adhering and remaining abrasive particles. For such a method, however, any polishing fluid which can effectively polish silicon wafers has not yet been developed. Meanwhile, chemical polishing using a conventional polishing fluid is vitally defective due to an impractically low polishing rate as compared with mechanochemical polishing using an alkaline colloidal silica polishing fluid.
Under such circumstances, a polishing method comprising two steps (hereinafter referred to as "two-step polishing method") has been proposed in, for example, Japanese Unexamined Patent Publications Nos. 64-18228 and 1-193170. In this method, the process for polishing a silicon wafer is divided into two steps, namely, a step for removing an oxide layer formed on the top surface of a silicon wafer and a step for polishing the surface newly appearing after removal of the oxide layer; and a polishing fluid containing abrasives is used for removal of the oxide layer, and a polishing fluid without abrasives is used for polishing the newly appearing surface.
In such a two-step polishing method, the polishing time can be shortened since mechanochemical polishing using a polishing fluid containing abrasives is employed for rough polishing to remove the oxide layer, and the abrasive particles do not remain on the resulting polished surface since disk-system chemical polishing using a polishing fluid without abrasives is employed as a finish polishing to polish the newly appearing surface. Recently, such a two-step polishing method is particularly attracting attention since it has the advantages of both mechanochemical polishing and disk-system chemical polishing.
The two-step polishing method, however, has some problems. For example, since the finish polishing after removal of the oxide layer is performed using a polishing fluid substantially free of abrasive particles, it takes an excessive and impractically long time as a conventional polishing fluid without abrasive particles is used.
The present invention has been accomplished under such circumstances, and the first object of the present invention is to provide a polishing fluid composition which contains a remarkably reduced amount of colloidal silica acting as abrasives, does not cause the problems of the related art, and can effectively polish surfaces of semiconductor silicon wafers or surfaces of films comprising silicon formed on silicon wafers. Additionally, the second object of the present invention is to provide a polishing fluid composition especially useful for the polishing step after the oxide-layer-removing step in the aforementioned two-step polishing method.
Further, another object of the present invention is to provide a method for effectively polishing surfaces of semiconductor silicon wafers or surfaces of films comprising silicon formed on silicon wafers by using the aforementioned polishing fluid composition of the present invention.